1. Field of the Invention
The present invention relates to mass analysis. More particularly, the present invention relates to sample preparation and handling for mass spectrometry processes.
2. Description of the Related Art
Mass spectrometry (MS) is a method of mass analysis in which the chemical composition of a substance is identified by separating gaseous ions from the substance according to their mass-to-charge ratio. One type of ionization used in mass spectrometry is known as matrix-assisted laser desorption/ionization (MALDI). In MALDI, a sample and a matrix are co-crystallized from a liquid solution. The resulting co-crystal is irradiated with a laser, which causes the matrix molecules to absorb some of the incident energy. The absorbed energy causes some of the matrix and sample molecules to ionize and desorb into the gas phase. Once the sample molecules have been ionized and desorbed into the gas phase, they are amenable to analysis by mass spectrometry.
A sample that contains a mixture of chemical compounds is frequently purified prior to analysis by mass spectrometry. One method of purification is by liquid chromatography (LC), in which the mixture of compounds to be separated is dissolved in a liquid phase and the liquid phase is passed over a stationary phase contained in a chromatography column. Compounds that interact more strongly with the stationary phase are retained for a longer period of time on the column, which permits a mixture of compounds to be separated based upon differences in retention times.
One means of further separating compounds with differing retention times is to fractionate the liquid phase as it exits the chromatography column. The formation of a large number of small amounts of the liquid phase produced by an LC is known as fraction collection. The precision and speed with which fraction collection is performed is critical to achieving high-quality separation and analysis. In addition, after purification by LC, the liquid phase containing the sample molecules can be combined directly with a matrix solution to enable MS analysis. Hereinafter, a liquid refers to any solution containing either sample molecules or matrix molecules, or a mixture of both. A sample refers to a portion of a liquid that has been prepared and provided for analysis, such as by deposition on a sample plate.
Mass spectrometry analysis is currently being applied to increasingly complicated molecules and mixtures. In addition, advances in medicine, science and technology have created a growing demand for sophisticated analytical tools. This demand, in turn, requires the development of methods for the rapid and efficient preparation, purification and analysis of samples by mass spectrometry. Methods that enable the purification of large numbers of samples by LC and the analysis of large numbers of samples by MS have been developed independently. The demand for a rapid and efficient means to prepare and handle large numbers of LC fractions for analysis by MS, however, remains unmet.
Typically, systems for automated fraction collection include single-channel devices that use either a touch-down cycle or piezo-electric dispensers for depositing a liquid sample. Touch-down devices use mechanics for physically spotting a liquid droplet onto a fraction collection plate. These devices normally use a narrow capillary connected to a column that holds the liquid. A tip of the capillary physically contacts the fraction collection plate to deposit a droplet of the liquid.
Touch-down devices are notoriously prone to misalignment, wear and breakage. Also, preservation of the chromatographic resolution becomes an issue in touch-down collection when small fractions are collected. A significant portion of the liquid deposited in one spot can be carried over to the next spot by sticking to the capillary tip. To remedy carry-over, a make-up flow can be added by teeing-in additional solvent, or by applying a sheath flow, or both. These remedies increase the collected volumes so that a proportionately smaller sample fraction is carried over. However, the larger volumes dilute the sample concentration applied to the fraction collection plate, which, in turn, leads to lower sample densities and lower signal-to-noise in the signal generated by the mass spectrometer.
A piezo-electric dispenser is a type of non-contact liquid droplet deposition device, in which a piezo-electric element is used to apply a pulse-driven pressure to a dispenser. The pressure forces, or ejects, a droplet of liquid out through a nozzle in the dispenser. Samples deposited using piezo-electric devices typically exhibit reduced chromatographic resolution because a larger dead volume is required to eject the droplet, and the concentration of sample molecules to matrix molecules is correspondingly reduced. Further, problems exist with keeping the dispenser nozzle clean if matrix solvent contains larger amounts of non-volatile material. Application of a sheath flow is not possible in this case.
Other methods of micro deposition of LC samples include electrospray and streaking. Electrospray is a non-contact sample deposition process, which is sensitive to solvent composition and flow rate and which spreads the sample out over larger areas. Streaking is a contact form of deposition, which requires flash evaporation or freezing to preserve chromatographic resolution. Both electrospray and streaking processes are sensitive to solvent composition.